Linepipe steel

ABSTRACT

A method of producing coil plate on a hot strip mill is disclosed. The method includes coiling hot rolled coil plate strip at a temperature that is selected (a) to minimise precipitation of Cr/Mo carbides or (b) so that any Cr/Mo carbides that form are sufficiently fine that they go into solution in any subsequent heat treatment of coil plate made from the strip.

The present invention relates to linepipe steel, a method of producinglinepipe steel strip, a method of producing linepipe from linepipe steelstrip, and a linepipe complying with the specified requirements of theAPI 5L Grades made from linepipe steel.

In Australia, the electric resistance welding (ERW) method is theprincipal method for linepipe manufacture. ERW requires steel strip feedthat has been sourced from a hot strip mill. ERW involves heating sideedges of strip to temperatures above the melting point of the steel andthereafter butting and thereby welding the side edges together.

Mn levels in linepipe steels produced by the applicant have historicallybeen in the range of 0.8-1.5 wt. %. The main purpose of the Mn additionis to provide solid solution strengthening.

However, one disadvantage of Mn is that it has strong segregatingtendencies that typically manifest in anomalous microstructures (withhigh hardness and low toughness) at the centreline of strip produced byhot rolling continuously cast steel slabs.

Such anomalous microstructures can have deleterious effects on themechanical properties of the ERW weld line particularly when linepipesare produced from centre slit strip feed. Specifically, the step ofbutting side edges of strip together in ERW diverts Mn rich centrelinesegregation bands at the slit side edge of strip into the plane of theweld that forms. The result is to compromise the toughness of the weldline.

Such anomalous microstructures arising from centreline segregation canalso have deleterious effects on welds produced by welding togetheraligned ends of linepipes during construction of a line using upsetwelding processes such as MIAB and flash butt welding. These processesinvolve induction heating the rims of the aligned ends by and thenbutting the ends together to produce autogenous welds. Currently theMIAB welding process, which has some operational and economicaladvantages over conventionally used manual welding of line pipe, is notused widely in pipe line construction.

The adverse effect of centreline segregation is exaggerated by thecoincident occurrence of elongated MnS inclusions in the strip. Theplasticity of MnS inclusions in the hot rolling process increasesdirectly with increasing Mn level. The detrimental effects of MnSinclusions on ductile fracture propagation resistance of pipeline steelsare well known. These inclusions exert a controlling influence on thefracture toughness of both the pipe body and the weld line. The adverseeffect on the weld line toughness is particularly evident for the caseof pipes made from centre slit strips.

Traditionally, the adverse effects of elongated MnS inclusions andcentreline segregation have been controlled by limiting S concentrationsin steel to be below 0.005% (or even lower limits) depending on thespecific requirements of a pipeline.

In addition, some steel manufacturers, as an additional countermeasure,have the capability to achieve complete sulphide shape control using Cainjection processes.

However, there are significant capital and operating costs associatedwith both of the above measures, and the measures are not attractiveoptions for these reasons.

The present invention provides an alternative solution that is based oncomposition selection of linepipe steel that should not involvesignificant additional capital and operating expenditure. In addition tomaking it possible to produce improved quality linepipe, the linepipesteel is also specifically suited to on-site upset welding processessuch as the MIAB and flash butt welding processes.

The solution is to use a steel having (a) a considerably reducedconcentration of Mn (typically no more than 0.50 wt. %, preferably nomore than 0.35%) than used conventionally for linepipe manufacture and(b) a small concentration of Ti (typically at least 0.01 wt. %).

Furthermore, for higher strength API linepipe grades, the solution mayinclude additions of Cr in the steel. The applicant has found thatalloying additions of Cr can also be effective in increasing thehardness and reducing the plasticity of MnS inclusions. In general, thespecification toughness requirements of linepipe steels increase withincreasing strength levels. For the higher strength API linepipe grades,the use of Cr additions in place of Mn will have the combined benefit ofcontributing to both increased strength and toughness.

The low Mn concentration reduces the degree of centreline segregationand therefore the anomalous microstructures that would otherwise beformed in the strip formed by hot rolling continuously cast slabs. Inaddition, the plasticity of MnS inclusions is greatly reduced at low Mnconcentrations. The inclusions are relatively hard and remain in alargely globular form in a hot rolled strip product. The Ti additionfurther enhances the hardness of MnS inclusions, whilst also assistingin achievement of improved surface quality and grain refinement in thesteel and associated weld heat affected zones.

According to the present invention there is provided a linepipe steelhaving the following composition, in wt %:

C: up to 0.18;

Mn: 0.10 to 0.50;

Ti: at least 0.01;

Si: up to 0.35;

Nb: up to 0.10;

Al: up to 0.05;

Ca: up to 0.005;

S: up to 0.015;

P: up to 0.020;

Cr: up to 1.0;

Mo: up to 0.5;

B: up to 0.002;

Ni: up to 0.35;

Cu: up to 0.35;

V: up to 0.06;

Fe: balance; and

incidental impurities.

The term “incidental impurities” is understood herein to mean impuritiesthat are the result of the steelmaking process and the feed materialsused in the steelmaking process and are not deliberate additions to thecomposition and are not already in the list of elements. Sn is one suchelement.

The linepipe steel includes Mn and Ti as deliberate additions to thecomposition.

The linepipe steel may also include additional elements as deliberateadditions to the composition.

Cr, Mo, B, Ni, Cu, and V are examples of additional elements.

Deliberate additions of elements may be required depending on themechanical properties required for linepipe made from the steel. Forexample, for high strength linepipe grades, such as API 5L X65 and X70,which traditionally rely on relatively high Mn concentrations forstrength purposes, Cr and Mo may be added to compensate for the lowconcentration of Mn. Furthermore, B may be added and be present in aprotected solute form to enhance hardenability. It is noted that, when Bis added, preferably the composition includes sufficient Ti to combinewith all of the N in the composition and, hence, avoid the formation ofBN.

In addition, deliberate additions of elements may be required dependingon particular requirements relating to the end-use applications of thelinepipe steel. For example, Ni and Cu may be required as elements inthe composition for sour service applications.

Typically the steel composition includes less than 0.10 wt. % C.

Typically the steel composition includes at least 0.02 wt. % C.

Preferably the steel composition includes at least 0.03 wt. % C.

More preferably the steel composition includes at least 0.04 wt. % C.

Typically the steel composition includes less than 0.35 wt. % Mn.

Typically the steel composition includes at least 0.15 wt. % Mn.

Preferably the steel composition includes at least 0.20 wt. % Mn.

More preferably the steel composition includes at least 0.25 wt. % Mn.

Typically the steel composition includes less than 0.05 wt. % Ti.

Preferably the steel composition includes less than 0.03 wt. % Ti.

More preferably the steel composition includes less than 0.04 wt. % Ti.

Typically the steel composition includes less than 0.25 wt. % Si.

Typically the steel composition includes at least 0.005 wt. % Si.

Typically the steel composition includes less than 0.08 wt. % Nb.

Typically the steel composition includes at least 0.001 wt. % Nb.

Preferably the steel composition includes at least 0.01 wt. % Nb.

Typically the steel composition includes at least 0.01 wt. % Al.

Typically the steel composition includes less than 0.001 wt. % Ca.

Typically the steel composition includes less than 0.012 wt. % S.

Typically the steel composition includes less than 0.01 wt. % S.

Typically the steel composition includes at least 0.005 wt. % S.

Typically the steel composition includes less than 0.020% wt. P.

Typically the steel composition includes less than 0.7 wt. % Cr.

Preferably the steel composition includes less than 0.5 wt. % Cr.

Typically the steel composition includes less than 0.3 wt. % Mo.

According to the present invention there is also provided a linepipemade from the above-described linepipe steel.

According to the present invention there is also provided a method ofproducing a coiled strip of the above-described linepipe steel that issuitable to be used as a feedstock for producing a linepipe, whichmethod includes the steps of:

(a) casting a slab of the above-described linepipe steel;

(b) hot rolling the slab to form a strip having a required thickness,typically 5-10 mm; and

(c) coiling the strip.

Preferably the microstructure of the linepipe steel in the coiled stripproduced by the above-described method is predominantly fine grainedpolygonal ferrite.

Preferably the microstructure includes a small (up to 15%) volumefraction of pearlite in the case of medium strength linepipe grades,such as API 5L X42 and X60.

Preferably the microstructure includes acicular ferrite and/ormartensite/austenite in the case of high strength linepipe grades, suchas API 5L X65 and X70.

According to the present invention there is also provided a method ofproducing a linepipe which includes electric resistance welding theabove-described linepipe steel strip and forming the linepipe.

Apart from the narrow seam annealed region of the ERW weld zone, themicrostructure of the linepipe is essentially unchanged by the pipeforming process and is the same as that of the linepipe steel in theabove-described linepipe steel strip.

The applicant has carried out research work that has evaluated theextent of centerline segregation in linepipes made from theabove-described electric resistance welded linepipe steel strip andlinepipes made from electric resistance welded, conventional high Mnlinepipe steel strip.

FIG. 1 illustrates the results of the research work.

FIG. 1 comprises two graphs. Each graph plots the concentration of Mn(measured by Electron Probe Microanalysis) in the particular steeltested against distance from the centerline of the steel strip.

The upper graph of FIG. 1 is for a conventional high Mn linepipe steelwith a Mn concentration of 1.1 wt. %.

The lower graph of FIG. 1 is for a linepipe steel in accordance with thepresent invention with a low Mn concentration of 0.3 wt. %.

It is readily apparent from a comparison of the two graphs that therewas significantly less variation in Mn concentration in the vicinity ofthe strip centerline for the linepipe steel in accordance with thepresent invention. This indicates significantly less segregation in thislinepipe steel. Consequently, the toughness of the ERW weld line forpipes made from centre slit strip can be significantly improved.

The improved toughness is illustrated by the results of further researchwork that are summarized in Table 1 below.

Table 1 provides the results of weld line Charpy V impact tests on“gull-winged” wall thickness specimens produced from 219 mm×6.4 mm pipemade from a conventional high Mn linepipe steel and a low Mn linepipesteel in accordance with the present invention. The tests were made withthe test specimens positioned with their notch locations coincident withthe weld lines. The chemical compositions of both the high Mn and thelow Mn steel pipes tested in this way are provided in Table 2.

TABLE 1 Weld Line Charpy V Impact Tests Charpy V Impact Test Pipe Energy(J) at 0° C. Steel Type Number (3 tests per pipe) Present Invention 1 82128 104 (0.08% C—0.38% Mn) 2 130 128 132 3 96 98 88 Conventional High Mn1 17 22 48 (0.08% C—1.07% Mn) 2 20 22 24 3 20 12 30

TABLE 2 Chemical Composition of Pipes Subject to Weld Line Charpy VTests (wt %) Steel C Mn Si S Al Nb Ti Ca N Low Mn 0.08 0.38 0.19 0.0040.029 0.018 0.021 0.0008 0.0043 (Invention) High Mn 0.08 1.07 0.33 0.0030.045 0.055 0.013 0.0007 0.0047 (Conventional)

The results in Table 1 are typical weld line Charpy test resultsobtained by the applicant in the research work.

It is evident from Table 1 that the linepipe steel of the presentinvention had weld line toughness consistently higher than for theconventional high Mn linepipe steel tested.

The research work carried out by the applicant investigated further theeffect of low Mn concentrations on the Charpy V impact energy oflinepipe steel strip in accordance with the present invention.

The results of the further research work are presented in FIG. 2.

FIG. 2 is a graph of Charpy V Energy at −15° C. versus the Mnconcentrations (in wt. %) of a number of linepipe strips in which the Ccontent was held in the range of 0.08-0.10% and the S content was variedover the range of 0.003 to 0.010%.

FIG. 2 shows that low Mn steels of the present invention can toleratehigher concentrations of S than higher Mn steels to obtain a giventoughness. This is an advantage from the viewpoint of the practicalissue of making steel with low S concentrations. In other words, it isapparent from FIG. 2 that the low Mn alloy design approach of thepresent invention permits considerably higher S concentrations to beused to achieve a given specification requirement for Charpy V impactenergy.

Many modifications may be made to the present invention described abovewithout departing from the spirit and scope of the invention.

1. A linepipe steel includes the following composition, in wt %: C: upto 0.18; Mn: 0.10 to 0.50; Ti: at least 0.01; Si: up to 0.35; Nb: up to0.10; Al: up to 0.05; Ca: up to 0.005; S: up to 0.015; P: up to 0.020;Cr: up to 1.0; Mo: up to 0.5; B: up to 0.002; Ni: up to 0.35; Cu: up to0.35; V: up to 0.06; Fe: balance; and incidental impurities.
 2. Thelinepipe steel defined in claim 1 includes less than 0.10 wt. % C. 3.The linepipe steel defined in claim 1 or claim 2 includes at least 0.02wt. % C.
 4. The linepipe steel defined in claim 3 includes at least 0.03wt. % C.
 5. The linepipe steel defined in claim 4 includes at least 0.04wt. % C.
 6. The linepipe steel defined in any one of the precedingclaims includes less than 0.35 wt. % Mn.
 7. The linepipe steel definedin any one of the preceding claims includes at least 0.15 wt. % Mn. 8.The linepipe steel defined in claim 7 includes at least 0.20 wt. % Mn.9. The linepipe steel defined in claim 8 includes at least 0.25 wt. %Mn.
 10. The linepipe steel defined in any one of the preceding claimsincludes less than 0.05 wt. % Ti.
 11. The linepipe steel defined inclaim 10 includes less than 0.03 wt. % Ti.
 12. The linepipe steeldefined in any one of the preceding claims includes less than 0.25 wt. %Si.
 13. The linepipe steel defined in any one of the preceding claimsincludes at least 0.005 wt. % Si.
 14. The linepipe steel defined in anyone of the preceding claims includes less than 0.08 wt. % Nb.
 15. Thelinepipe steel defined in any one of the preceding claims includes atleast 0.001 wt. % Nb.
 16. The linepipe steel defined in claim 15includes at least 0.01 wt. % Nb.
 17. The linepipe steel defined in anyone of the preceding claims includes at least 0.01 wt. % Al.
 18. Thelinepipe steel defined in any one of the preceding claims includes lessthan 0.001 wt. % Ca.
 19. The linepipe steel defined in any one of thepreceding claims includes less than 0.012 wt. % S.
 20. The linepipesteel defined in claim 19 includes less than 0.01 wt. % S.
 21. Thelinepipe steel defined in any one of the preceding claims includes atleast 0.005 wt. % S.
 22. The linepipe steel defined in any one of thepreceding claims includes less than 0.7 wt. % Cr.
 23. The linepipe steeldefined in claim 22 includes less than 0.5 wt. % Cr.
 24. The linepipesteel defined in any one of the preceding claims includes less than 0.3wt. % Mo.
 25. A linepipe made from the linepipe steel defined in any oneof the preceding claims.
 26. A method of producing a coiled strip of thelinepipe steel defined in any one of the preceding claims that issuitable to be used as a feedstock for producing a linepipe, whichmethod includes the steps of: (a) casting a slab of the linepipe steeldefined in any one of the preceding claims; (b) hot rolling the slab toform a strip having a required thickness, typically 5-10 mm; and (c)coiling the strip.
 27. The method defined in claim 26 wherein themicrostructure of the linepipe steel in the coiled strip ispredominantly fine grained polygonal ferrite.
 28. The method defined inclaim 26 or claim 27 wherein the microstructure includes a small (up to15%) volume fraction of pearlite in the case of medium strength linepipegrades, such as X42-X60.
 29. The method defined in claims 26 or claim 27wherein the microstructure includes acicular ferrite and/ormartensite/austenite in the case of high strength linepipe grades, suchas X65 and X70.
 30. A method of producing a linepipe which includeselectric resistance welding the linepipe steel strip defined in any oneof claims 26 to 29 and forming the linepipe.